EDIT: ignore this; I was confused / misinformed

It's about pH. CO2 creates carbonic acid when it dissolves in water. Your blood pH, in turn, controls how much you feel like you need to breathe. So with high CO2, your respiration rate slows down, and that can lead to low oxygen levels.

Note that the physiology and biochemistry of this is complicated (e.g. blood is a very good pH buffer and it's actively regulated by kidneys etc) and it's very much a nascent field of research, so I think AI will be overconfident and hallucination-prone.

Source: I worked in high-co2 caves for my PhD so have read about this a lot. I always carried a CO2 monitor. Our rule was to get out if we saw 20,000 ppm or greater. I spent thousands of hours above 10,000ppm.

My medical student flatmates were talking a lot about acidosis and alcalosis :)

It was the first time that I heard about them. These basically never happen if your body and environment are halfway decent, but they are important in exceptional situations.

Wouldn't high CO2 make you breath faster?

You are right about the pH implications, but respiratory acidosis leads to hyperventilation, not hypoventilation. CO2 will kill you regardless of oxygen supply.

https://en.wikipedia.org/wiki/Hypercapnia